Integrating device for use in industrial process control apparatus

ABSTRACT

An industrial process control system wherein a digital data processor controls process conditions in a time shared manner by periodically sampling the conditions and then communicating in sequential order with the process regulating means by use of a series of constant amplitude command signals whose time duration is proportional to the change to be made in the process regulating means and a device for translating a command signal of this type to a suitable control signal for changing the setting of the process regulating means.

United States Patent [72] inventor Robert J. Robinson Lexington, Mass. [21 Appl. No. 749,230 [22] Filed May 16, 1968 Division of Ser. No. 500,612, Oct. 22, 1965 Pat. No. 3,441,053, Apr. 29, 1969. [45] Patented May 4, 1971 [73] Assignee The Foxboro Company Foxboro,Mas.

[54] INTEGRATING DEVICE FOR USE IN INDUSTRIAL PROCESS CONTROL APPARATUS 13 Claims, 15 Drawing Figs.

[52] U.S.Cl 235/183, 188/88, 188/965, 235/200 [51] Int. Cl G06g 7/18, F16k31/l43 [50] Field ofSearch 235/183,

200, 201; 318/18; 317/123, (Inquired); 310/30, 35, 37,335/(lnquired); 336/110; 188/965, 88 (E); 91/361--363, 459

[56] References Cited UNITED STATES PATENTS 2,327,722 8/1943 Kouyoumjian 188/965 2,701,331 2/1955 Holst 310/35X 3,026,892 3/1962 Tsien 9 l /459X 3,039,566 6/1962 Rumsey 188/8815 3,136,224 6/1964 Escobosa 91/363 Primary Examiner-Malcolm A. Morrison Assistant Examiner-Felix D. Gruber Attorney-Bryan, Parmelee, Johnson & Bollinger ABSTRACT: An industrial process control system wherein a digital data processor controls process conditions in a time shared manner by periodically sampling the conditions and then communicating in sequential order with the process regulating means by use of a series of constant amplitude command signals whose time duration is proportional to the change to be made in the process regulating means and a device for translating a command signal of this type to a suitable control signal for changing the setting of the process regulating means.

i S 0 i t t t PATENTEDQAY 41971 SHEET 2 BF 4 PATENTED HAY 41971 SHEET 3 0F 4 INTEGRATING DEVICE FOR USE IN INDUSTRIAL PROCESS CONTROL APPARATUS This is a division of application Ser. No. 500,612, filed Oct. 22, 1965, now US. Pat. No. 3,441,053.

This invention relates to industrial process control systems. More particularly, this invention relates to such systems in which a time-shared digital data processor is utilized to develop control signals for operating a plurality of process valves or other process regulators.

- In recent years, there has been increasing use of high-speed digital data processors or computers" with industrial process control systems for controlling process conditions such as pressure, temperature and flow rate. To make most effective use of the computing capacity of the data processor, it is desirable that the processor be operated in a time-shared multiplex mode; that is, a mode in which the various process conditions to be controlled are sampled periodically, in a predetermined sequence. As each sample is taken, the computer performs appropriate computations to develop a corresponding output command signal thereby producing a series of command signals each indicating the change to be made in the setting of a corresponding valve or other process regulating means. Such systems have been referred to as direct digital control" systems, to distinguish from arrangements wherein a digital computer is used simply to adjust the set point of a conventional analog controller.

Advantageously, the data processor in a direct digital control system is arranged to'develop for each valve a command signal having a constant magnitude and atime duration proportional to the change to be made in the valve position. Since valves and other proces regulators typically are not capable taining each valve control signal at the computed value during the times when the processoris carrying out computations for the other valves; that is, drift of the valve control signal over a period of time shouldbe zero or at least very small.

It is the principal object of the present invention to provide an improved system of the type described above. A more specific object of this invention is to provide such apparatus which isrelatively compact, economical to construct, and stable in operation. Other objects, aspects and advantages of the invention will be pointed out in or apparent from the following description considered together with the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram representing a process control system constructed in accordance with the present invention;

FIG. 2 is a perspective view, partially in section and partially broken away, of a component of the system shown in FIG. 1;

FIG. 3 is a vertical section of the electromagnetic currentresponsive device shown in FIG. 2;

FIG. 4 is an enlarged, partially broken-away view of a portion of the structure shown in FIG. 3;

FIG. 5 is a graph illustrating the variation'of various operational parameters of the device shown in FIG. 3;

FIG 6 is a'schematic diagram showing typical waveforms and time sequences of various command signals produced in the apparatus shown in FIG. 1;

FIG. 7 is a perspectiveview of a modified signal translating unit;

FIG. 8 is a vertical section taken along line 8-8 of FIG. 7;.

FIG. 9 is a partially cross'sectional and partially brokenaway view of a modified version of the FIG. 3 structure;

FIG. 10 is a cross section taken along line 10-10 of FIG. 9;

FIGS. 11A and 11B are schematic diagrams illustrating certain operational features of the disclosed apparatus;

FIGS. 12A and 12B are schematic diagrams illustrating certain operational features of a modified system; and

FIG. 13 is a schematic circuit diagram of a valve position indicator circuit.

Referring now to FIG. 1, there is shown in the upper lefthand comer a series of input signal lines 15 eachconnected to a condition-sensing device (not shown) such as a flow transmitter, temperature sensor or the like, which is adapted to produce a measurement signal indicating the magnitude of the corresponding process condition being measured. Each measurement signal, which may for example be a DC signal in-the range of l0-50 milliamps, is conducted by a corresponding one of the lines 15 to a high-speeddigital data processor 16.

This processor 16 includes an input interface means comprising a multiplexer I6a which operates to sample. periodically all of the incoming measurement signals in a predetermined sequence. These sample signals are directed in turn to the main computation section of the data processor, and this section functions in conventional ways'to-carry out the programmed arithmetic operations with respect to each of the measurement signals. For each such computation the data processor produces a corresponding output in the form of a pulse-type command signal having a fixed magnitude and a time-duration proportional to the computed change to be made to the process valve.

These command signals are distributed, through an output interface means comprising a multiplexer 16b to output line 17 each leading to a signal translating unit 18. This unit includes'means to be describedbelow, for producing a pneumatic valve control signal which is sent through a line 19 to a conventional process valve 20 which changes the valve position in response to the pneumatic signal it receives.

FIG. 6 illustrates two successive cycles of typical command signals as delivered over lines 17 Each of the pulses identified by means of one of the capital letters A-E represents a command signal transmitted over one of the multiple output lines 17. These command signals have either a positive ornegative polarity, depending upon which direction the valve is to be moved.

Referring now to FIGS. 2 and 3, which show the construction of signal translating unit 18in detail, the command signal on each line 17 is directed through a pair of wires 40 and 42 to a pair of solenoid coils 36 and 38 of a signal transducer 26 adapted to develop an output movement of a mechanical member over a distance which is an approximately linear function of the time duration of each command signal. Thus, the device 26 produces a movement of its output shaft 48 of constant velocity during each command signal.

Within the interior of the solenoid coils 36 and 38 is a hol-' low tube 34 containing a viscous liquid 32. Slidably mounted within this tube is a plunger 30 basically comprising a cylindrical permanent magnet 102 fitted with endpieces 104 and 106, the latter having. circumferential sealing means 108. A restricted liquid passageway extends through the interior of the plunger. A full explanation of the features of thisliquid passageway will be set forth in detail hereinbelow.

Coils 36 and 38 are connected in series but are reversewound to produce flux in' opposite directions with respect to the'longitudinal axes of .the coils. A command signal of one polarity causes the plunger to move in one direction, while a command signalof opposite polarity causes the plunger to move in theopposite direction. For'relatively small displacements of the plunger from its central position, the force developed by the coils is approximately proportional to the magnitude of the current flow to the coils. Since this current flow is substantially constant during each command signal, the force developed by the coilsis very nearly constant for the duration of the'command signal.

The movement of the plunger 30 in response to the solenoid-produced'motive force is impeded by the resistance to the flow of the liquid 32 through the restricted passageway of the plunger; At a given fluid temperature, the impedance produced by this dashpot" effect is such as to cause the plunger to move at a velocity which is a function of the magnitude of the force on the plunger 30, and, hence, is a function of the magnitude of the current flowing through the solenoid coils.

constant, sothat plunger 30 moves with a substantially constant. velocity. Thus, the distance traveled by the plunger will be proportional to the time duration of the corresponding command signal.

' Now considering the structure of signal translating unit 18 in greater detail, plunger 30 is secured, at its upper end, to a link 44 which in turn is fastened by a ball joint 46 to an output shaft 48 extending upwardly from the top of the transducer 26. As shown particularly in FIG. 2, this output shaft 48 is connected through a linkage arrangement to a mechanism for producing an output pressure signal corresponding to the vertical position of shaft 48.

The mechanism for converting the position signal provided by shaft 48 into a pneumatic signal includes a linkage arrangement 54 which is connected to shaft 48 by means of a ball joint 50 and lever arm 52. Linkage 54 is connected to a pneumatic transmitter 56 which in turn is connected to a pneumatic relay 58. Pressurized air is supplied to the relay 58 by means of an air line 61, and a pneumatic output signal emerges from relay 58 through the output line 19 which conducts the output pneumatic signal to the valve (see FIG. 1). Changes in the vertical position of the plunger 30 cause directly proportional changein the magnitude of the pressure of the signal supplied to line 19. r

The signal translating unit 18 includes a metal housing 55 whose cover is not shown in the drawings. Signal transducer 26 has a rectangular base 62 with four threaded mounting holes 68 into which screws are fitted to secure the device to the bottom of the case 55. The pneumatic relay 58 also is secured to the bottom of the case 55, with the hoses l9 and 61 and their fittings extending through holes in the case bottom.

A mounting plate 51 is secured to a channel-shaped bracket 53 which is welded at its edges to the cylindrical barrel 64 of the device 26. Linkage 54, pneumatic transmitter 56 and a potentiometer 87 all are mounted on plate 51. Plate 51 is not secured to the case' 55. Thus, the components mounted on plate 51 are rigidly and securelymounted with respect to the device 26 with little possiblity of misalignment due to external forces applied to the case.

Now considering the linkage 54 in detail, the left end 63 of lever 52 moves up and down between two nuts 65 and 67 on a bolt 69 which is welded to a bracket 71 attached to the mounting plate 51. This arrangement provides an adjustable stop structure for limiting the extent of vertical movement of the plunger shaft 48 and plunger 30.

Lever 52 is rigidly secured at its right end to a shaft 73 which is rotatably mounted at one end in mounting plate 51, and at the other end in boomerang-shaped bracket 75. Bracket 75 is secured to mounting plate 51 by means of a pair of studs 77.

Also attached rigidly to shaft 73 is another lever 79 to the end of which is attached an eccentn'cally mounted cylindrical counterweight 81. Counterweight 81 compensates for the weight of the plunger and other components connected to lever 52 and makes the device sensitive to small forces applied to the plunger 30. Counterweight 81 is mounted eccentrically on lever 79 so as to be adjustable to adjust the amount of torque it applies to shaft 73.

Also connected rigidly to shaft 73 is another lever 83 which is connected at its right end to the wiper arm 85 of the potentiometer 87. The potentiometer 87 has a pair of electrical leads 89 which are connected to a meter (not shown) or other means for indicating the magnitude of the signal provided by the position of wiper arm 85 which is mechanically linked to plunger shaft 48.

The friction between wiper arm 85 and the body of potentiometer 87 is used to advantage as a brake to hold the linkage 54 and, hence, the plunger 30 and the output signal of converter 24 at a constant level between successive command signals, thus assuring that the output of the signal translating unit does not drift."

Also secured to shaft 73, at its rearmost end, is another lever 91 which is pivotally connected to a horizontal lever 93, which, in turn, is pivotally connected to a flapper lever 95 of the pneumatic transmitter 56.

Transmitter 56 is a known device, but its operation will be described here briefly for the sake of completeness. Flapper lever 95 is rigidly secured to a shaft 97 which is pivotally mounted in a bracket 99. Rigidly secured to shaft 97 is a "flapper 101 which is made of thin, flexible metal. The lower end of flapper 101 covers the outlet opening of an air nozzle 103 which is supplied with pressurized air from relay 58 by means of a hose 105. An air bellows 107 is mounted between a vertical support plate 109 and the central portion of the bracket 99. Bellows 107 is supplied with pressurized air from relay'58 by means of another flexible tube 111. The bracket 99 and the support plate 109 are interconnected by a baseplate 113, and the bracket 99 is pivotally mounted on baseplate 113 by means of a flexure 115.

When plunger shaft 48 of device 26 moves to a new vertical position, shaft 73 is rotated by means of lever 52, and the position of potentiometer wiper 85 is changed vertically to vary the indicator signal. Lever 91 rotates with shaft 73, and, by means of levers 93 and 95, moves the flapper 101 either away from or closer to the outlet opening of the nozzle 103.

Relay 58 also is well known and is described in US. Pat. No. 3,105,508. The relay includes a line restrictor (not shown in the drawings, but of the type ordinarily used in flapper-nozzle control devices) interposed between the air supply line 61 and the hose 105, so that the back pressure of the nozzle 103 will be controlled by positioning of the flapper 101 with respect to the outlet opening of nozzle 103. A pneumatic signal whose magnitude is directly proportional to the nozzle back pressure is delivered to the outlet hose 19 and to the bellows 107 by way of hose 111. The bellows 107 swings the bracket 99 either toward or away from the opening of nozzle 103 in order to locate the flapper 101 to within a few ten-thousandths of an inch of its position before the change of the position of the shaft 48. The small change in position of the flapper is sufficient to change the output signal delivered to hose 19 by a substantial amount. By these means, the output air hose 19 delivers a pneumatic output signal which is directly proportional to the position of shaft 48 and plunger 30 and therefor the change in the output signal is proportional to the time duration of the corresponding command signal.

The structural details of the unit 26 now will be described, again with reference to FlG. 3. The barrel 64 of the device 26 comprises a metal sleeve welded at its bottom to the base member 62. Base member 62 is provided with an internally threaded nipple 70 at its lower end which is adapted to receive an explosion-proof conduit 72 (see FIG. 2) for encasing the wires 40 and 42 and leading them to a safe room remote from unit 26, thus providing for explosion-proof operation of the .device. The wires 40 and 42 pass through one of a pair of holes 72 and 74 in base member 62 and are secured to two electrical tenninals 76, only one of which is shown in FIG. 3. The lower end of tube 34 is cemented by means of epoxy resin onto a projecting portion of the base 62. A washer 82 of insulating material surrounds the tube 34 near its base and the terminals 76 are secured to the washer 82. The spaces 78 surrounding the wires 40 and 42, the terminal 76, and the junction of the tube 34 with the base member 62 are encased or potted" in epoxy resin. Epoxy resin also fills the spaces 78 between the outer sleeve 64 and the coils 36 and 38 so as to insulate the wires 88 of the coils from surrounding components of the device. A thin insulating washer 84 separates the coils 36 and 38 from one another, and a thin sleeve 86 (see FIG. 4) of insulating plastic material sold under the trademark Mylar insulates the coils 36 and 38 from the tube 34.

This structural arrangement has the advantage that it is made explosion-proof without providing it with a heavy cover. The coils 36 and 38 are encased in epoxy resin, and the joint between the tube 34 and the base member 62 is separated from the atmosphere at the top of the integrator by a column of liquid approximately 3 inches in length. The electrical input.

terminals thus are well protected from the atmosphere and the device need not be encased in a heavy housing to meet the standards set for explosion-proofdevices.

A cap structure 66 of the spillproof" type .is provided for the unit 26. The cap structure 66 includes a spacermembcr 90 in the form of a cup with an opening in the bottom and bottom sides 92 sloping downwardly toward the opening. This opening forms the top opening for the tube 34. An inner cap member 94 is secured to the top edge of spacer 90 between a gasket 96. and a shield washer 98 by means of a retainer which screws onto external threads at the top of sleeve- 64. The downwardly depending sleeve portion of inner cap member 94 prevents most of the liquid from spilling out of the. tube if it is tipped over. When the device is returned to its vertical position, the liquid 32 flows back into tube 34.

Referring now to FIG. 4 as well as FIG. 3, the plunger 30 has a number of advantageous features. Plunger .30 comprises a hollow, thick-walled, tubular-shaped permanent magnet 102' with a pair of brass endpieces 104 and 106 which are affixed to the magnet 102. Magnet 102 can be made of any-known plunger 30 is permanently magnetized and, in effect, already,

saturated with magnetic flux. Thus, the rate offlux decrease in tolerable limits throughout a considerable distance away from the central position.

Referring again i to FIG. 4, .the ,lower endpiece 1060f plunger 30 has a circumferential groove into which-a plastic piston ring 108 is fitted. Ring 108 ensures that there willbe-a fluidtight 'fit between the plunger 30 and thewall of tube 34. Also, the ring 108 provides resistance to movement, of the. plunger and, together with the frictionofwiper arm 8510f. potentiometer 87 ,"provides a braking action -which holds the.

plunger in place between successive command signals, thus. protecting against drift;"

Endpiece'106 has centrally located axial hole ofa diameter smaller than the hole in the magnet 102. lnto thishole is fitted a capillary tube 110 of a predetermined inside'diameter-and length. The fluid 32 .flows through this tube when the plunger 30 moves throughthe fluid.

The upper endpiece 104 has a hole 112 in its uppennost end 0 and an enlarged recess 114 communicating with the hole 112. A jewel orifice member 116 is fitted into the recess 114. Orifice member 1l6has a tiny, centrally located hole 118 in a region of reduced thickness, thusforming a sharp-edged orifice.

A second capillarytube .120 of predetermined length and inside diameter passesthrough and is fitted into the upperend of endpiece 104. The inside diameter of tube-120 preferably is considerably smaller than that of the lower capillary tube 110'.

'30 the plunger as it moves away from its central position is relatively low. As a result,.the force variation remains well within When force is applied to the plunger 30, the fluid 32 in the tube 34 is forced through capillary tube 110 atone endof the plunger, and throughthe parallel passages provided by the capillary tube 120 and orifice 118. This arrangement provides advantageous temperature compensation for the device so that the velocity of travel of the plunger 30 through the fluid,

32 under. a constant applied force willrernain substantially constant despite wide variations in the temperature of the fluid.

In many dashpots, the fluid flows between the piston and cylinder walls. With this arrangement, the fluid flowis essentially laminar; that is, the fluid flows with low Reynolds'numbCI'S and little or no turbulence. When such flowconditions prevail, the velocity of the piston is an inverse function of the viscosity of the fluid through which, it moves..Unfortunately,

the viscosityof even the best dashpot. fluids available 'varies greatlyawith temperature. For example,'.one fluid which has been used is a 5 centistoke dimethylpolysiloxane fluid sold by Dow-Comingunder the designation DC 200. Although this i is believed to be one of the best available dashpot fluids, the

viscosity of such fluid is approximatelyZ centipoises at negative 25 F. and 28 centipoises at positive 1.7 5 F., a variation of over-l0 to l. The change in plunger velocity created by such a great viscosity change is not tolerable.

The provision of orifice member 116 with its sharp-edged orifice 118 greatly reduces the variation of plunger velocity with temperature. Essentially, it causes the fluid to flow turbulently instead of laminarly. With turbulent flow, the plunger velocity is essentially independent of the fluid viscosity and, instead; is an inverse function of the square root of the density of the fluid. The density of the fluid varies much less with temperature than does its viscosity.

FIG. '5 is a graph in which the velocity of plunger 30 is plotted as a function of temperature, with a constant voltage applied to the coils 36 and 38 at alltimes. The curve X shows the plunger velocity variation when the fluid flowis only through 'the orifice 116; that is, with capillary tube removed and capillary tube 120.blocked.-This curve shows a variation of plunger velocity of only about 18 percent for a 200 F. temperature range, a vast improvement over the l0 to l or 1,000 percent variation which would have been experienced with laminarfluid flow past the plunger.

Curve-X would-have a relatively small positive slope instead of the negative slope shown in FIG. 5 if there-did not exist another source of variation in plunger velocity with temperature. As the temperature of the coils 36 and 38 increases, their resistance alsoincreases. Thus, with command signals of constant voltage as shown inFlG. 6, the current flow to the coils tends to decrease with increasing temperature, thus decreasing the force applied to the plunger. This tends to'reduce the velocity of the plunger. Thus, the coil resistance change causesthe negative slope of the .curve X.

It should be noted that the velocity of fluid flow through the orifice 116 is proportional to the square root of the pressure applied tothe fluid. Since .this pressure is proportionalztothe force applied to the plunger, the flow velocity, and, hence, the

plunger velocity is proportional to the square root of the appliedforce. Thisis advantageous in that as the applied force changes with plunger position and temperature changes, the plunger velocity-changes by a lesser factor proportional to the square root of the force change.

The parallel capillary tube is provided to compensate for the negative characteristic of curve X; that is, to compensate for the effects of the decrease in fluid density and increase incoil resistance with increasing temperature. The flow through the capillary tube 120 is laminar and the velocity of flow through tube .120 is inversely proportional to viscosity of the fluid. The total flow through the piston 30 is the" sum of the flow through the orifice 118 and the capillary tube 120'. The composite velocity'curve obtained when both the capillary tube 120 and the orifice 118,are used is designated Y in FIG. 5. The fluid density-and coil resistance. variations give the velocity curve-a mild negative slope, and the capillary tube 120 gives curve Y a positive slope. The total velocity variation capillary tube 120 and thus reduces the flow velocity through those openings. Since the flow through capillary tube. 110 is laminar, theamount of pressure drop along tube 110 is directly proportional. to viscosity, and tube 110 causes the plunger velocity to tend to decrease with increasing temperatures.

By properlyv proportioning the relative dimensions of capil larytubes 110-and .120-and orifice 118,- the plunger velocity can be: made .very nearly constant with respect to temperature.

In FIG. 5, curve Z shows the results of using both capillary tubes 1 10 and 120 and orifice 118; The total velocity variation has been found to be only about one-half of l percent for a 200 F. temperature variation.

Typical dimensions of the flow passages described above now will be given for a model of the invention which has been successfully tested. The orifice 118 has a diameter of 0.0092 inch, and the central hole in the magnet 102 has a diameter of 0.156 inch. Series capillary tube 110 has an inside diameter of 0.050 inch and a length of l.ll inch, and the parallel capillary tube 120 has an inside diameter of 0.009 and a length of 0.538 inch.

An alternative electromagnetic unit 122 is shown in FIGS. 7 and 8. Unit 122 is similar in certain respects to unit 26 shown in FIGS. 3 and 4, and corresponding reference numerals are used for corresponding components in the two structures. The

a main differences between the units 26 and 122 are, first, that in the latter the dashpot fluid 32 does not flow through the plunger 30 when the plunger moves in the tube 34, but instead is fed around the plunger through a bypass passage. Also, a valve is provided for adjusting the flow rate through the bypass passage and thereby adjusting the ratio of input current to plunger velocity. Further, there is no orifice or capillary tube arrangement for temperature compensation. In addition, the plunger 30 has a composite structure which will be described hereinbelow.

Referring particularly to FIG. 8, electromagnetic unit 122 includes a base member 124, a top member 126, a bypass tube 128, and a coil structure. The coil structure includes oppositely wound coils 36 and 38, and is substantially the same as in the unit 26 shown in FIG. 3.

Top member 126 has a horizontal hole 131) which communicates with the upper portion of tube 34, and bottom member 124 has a similar horizontal hole 134 which communicates with the bottom of tube 34. The horizontal holes 130 and 134 communicate with vertical holes which are connected together by the bypass tube 128 to form a bypass conduit between the top and bottom of tube 34. The bottom and top members and the coil sleeve M are held together by a pair of bolts 136 and 138 (see both FIGS. 7 and 8). Threaded holes 131 and 133 are provided, respectively, in top and bottom members 126 and 124 for mounting the integrator 122 on a support. A metal ball 135 is fitted into the end of each hole 131 and 133 to close the hole.

The fluid bypass passage is provided with a needle valve M0 which comprises a threaded shaft 142 with a slotted head 144 and a pointed tip 146. The threaded shaft 142 engages threads in top member 126 and is shown in its most inward position in which it completely closes the bypass conduit at its tip and thus completely blocks the flow of fluid in the conduit. The slotted head 144 may be turned to withdraw the tip 146 and provide an adjustable opening for varying the rate of fluid flow through the conduit. In this manner, the velocity of the plunger 30 can be adjusted so as to increase and decrease the amount of movement of the plunger 30 for a given magnitude of electrical current input.

The plunger 30 shown in FIG. 8 is composed of two permanent magnet end-portions 1 and 150 which are secured to a central soft-iron portion 152. The plunger stem 48 is secured to the top of magnet 143 by means of an end plate 154. A ball-joint coupling 156 is attached to the upper end of shaft 48.

The solenoid structure consisting of the coils 36 and 38 together with the composite plunger 30 provides a much more nearly constant force against the resisting fluid than the solenoid shown in FIG. 3. The reason for this can be understood best by referring to FIGS. 11 and 12.

FIG. 11A shows, schematically, a solenoid structure with a single permanent magnet plunger 30 located centrally with respect to the oppositely-wound coils 36 and 38. FIG. 11A also includes three curves 1, 162, and 164a. These curves are qualitative graphs illustrating the variation of magnetic flux and forces along the length of the solenoid.

Curve 160a shows the variation of airgap radial flux density along the permanent magnet plunger; that is, the variationof the radial component of the flux in the airgap between the magnet and the wall of its enclosure. The flux density varies approximately linearly from the north pole to the south pole, and is zero at a point midway between the north and south poles.

Curve 162 shows the ampere-turns per unit of coil length for the coils 36 and 38 with equal currents flowing in each coil. The flux developed by coil 36 is of a polarity opposite to that in coil 38.

Curve 164a shows the variation of the force applied to the plunger. The force at any particular horizontal point is proportional to the product of the coil ampere-turns times the permanent magnet airgap flux at that point. Accordingly, at the center of the magnet, the applied force is zero. The total force applied to the plunger 30 is the summation (that is, the integral with respect to X, the distance along the solenoid) of the forces indicated in FIG. 164a.

FIG. 118 shows the same solenoid as in FIG. 11A, but with the plunger 30 moved to the right of'center by a distance m. Curve 1601; is identical to curve 160a except that all points have been moved to the right by a distance m. The curve 162 of the coil ampere-turns per unit of coil length remains unchanged and is not repeated in FIG. 118. Since the plunger airgap flux in the region m is negative and the ampere-turns per unit of coil length is positive, the force component 165 produced in that region is negative and opposes the force applied along the remainder of the length of the solenoid. This reduces the total amount of force available by an amount proportional to the square of the distance m moved by the plunger in the coil.

FIG. 12A shows a solenoid similar to that shown in FIG. 11A except that the plunger has the structure shown in FIG. 8; that is, the plunger has a central soft-iron portion 152 sandwiched between two permanent magnet members 148 and 150.

The permanent magnet airgap flux variation is shown in curve 166a, which is basically identical to curve 160a except that the flux density is approximately zero near the soft-iron piece 152.

The variation of coil ampere-turns per unit length is indicated by curve 168, and is substantially identical to that indicated in FIG. 162.

The force-variation curve for the solenoid is shown in curve 170a and is substantially the same as that shown in curve 1640 except that no force is produced in the soft-iron core portion because the flux in the airgap-near that portion is zero.

FIG. 128 shows the permanent magnet airgap flux density curve 166]), and the force curve 17% after the composite plunger has moved a distance m which is approximately equal to one-half of the length of the soft-iron portion 152. The presence of the soft-iron portion maintains the permanent magnet airgap flux at a constant zero level along its length and avoids the creation of a negative force component as long as the plunger moves away from a central location by a distance no greater than m. Thus, the composite plunger provides a range of plunger movement over which there is little force variation, thus increasing the accuracy of the electromagnetic device 122 in which the composite plunger is used.

The electromagnetic device 122 shown in FIG. 7 and 8 is advantageous not only in that its composite plunger has a relatively more constant force-variation curve, but also in that it is relatively less sensitive to errors caused by temperature changes than it would be if the bypass fluid conduit were not provided. A considerable portion of the fluid 32 is not closely in contact with the casing around coils 36 and 38 and thus is not subject to heating which might be caused by the electrical heating of these coils.

It should be understood that the composite plunger shown in FIG. it can be made with'a hole in it as in FIG. 3, and can include the temperatnre"compensation arrangements shown in FIG. 3. It also should be understood that a solid single-magnet plunger may be used in the FIG. 3 structure, if desired.

FIG. 9 and illustrate a modified plunger in which the fluid flows between the plunger and the walls of the plunger tube, thus eliminating the need for a bypass conduit as in FIGS. 7 and 8, or a hollow permanent magnet as in FIGS. 3 and 4. The plunger shown in FIGS. 9 and 10 comprises a cylindrical permanent magnet 172, two nonmagnetic guide-plates 174 and 176, with a plunger stem 177 secured to the uppermost plate 174. Each plate 174 and 176 has a diameter such that it fits into tube 34 in fluidtight sliding contact with the interior wall of tube 34. As is shown in FIG. 11, each plate 174 or 76 has four symmetrically positioned notches 178 in its edge so as to provide small fluid flow passages through which the fluid 32 can pass from one end of the plunger to the other.

FIGS. 9 and 10 illustrate a modified plunger in which the fluid flows between the plunger and the walls of the plunger tube, thus eliminating the need for a bypass conduit as in FIGS. 7 and 8, or a hollow permanent magnet as in FIGS. 3 and 4. The plunger shown in FIGS. 9 and 10 comprises a cylindrical permanent magnet 172, two nonmagnetic guide-plates 174 and 176, with a plunger stem 17'] secured to the uppermost plate 174. Each plate 174 and 176 has a diameter such that it fits into tube 34 in fluidtight sliding contact with the in- LJflOl' wall of tube 34. As is shown in FIG. 11, each plate 174 or 176 has four symmetrically positioned notches 178 in its edge so as to provide small fluid flow passages through which the fluid 32 can pass from one end of the plunger to the other.

The guide-plates 174 and 176 align the plunger centrally within the tube 34 so that the fluid flow will be symmetrical around the plunger. If such alignment means were not provided, the plunger would tend to be closer to one side of the tube than the other, especially under the influence of the electromagnetic forces applied to the plunger. ,Such misalignment would cause the flow-passageway to be asymmetric, and instability and/or errors in the movement of the plunger through the fluid would result.

FIG. 13 shows a unique arrangement for measuring and indicating the position of the plunger of the electromagnetic dashpot integrator. This arrangement utilizes the coils 36 and 38 and the plunger 30 as a diflerential inductor. The coils 36 and 38 are connected in a bridge circuit arrangement includ ing two identical resistors 180 or capacitors 182. Each coil 36 and 38 and each impedance element (resistor or capacitor) forms an arm of the bridge circuit. An alternating-current source 186 is connected to opposite junctions between the impedance elements and the coils, and an altemating-current voltmeter 184 is connected between the other junction of the bridge.

When the plunger 30 is located centrally with respect to the coils, the inductance of each coil, and, hence, the impedance of each side of the bridge circuit is the same, the bridge is balanced, and the voltmeter reading is zero. However, movement of the plunger increases the inductance of one coil while decreasing the inductance of the other coil by a corresponding amount, thus unbalancinga the bridge circuit. The reading on voltmeter 184 is a function of the relative position of plunger 30.

The capacitors 182 may be used instead of resistors 180 in order to prevent excessive loading of the command signal. Other appropriate circuitry can be provided for preventing undesired interaction between the command signal circuit and the AC source 186. The frequency of the AC source signals should be high enough to ensure that the AC signals do not move the plunger 30.

The position-indicating signal provided by either the potentiometer 87 or the bridge circuit shown in FIG. 14 can be used as a feedback signal to condition an analog backup controller serving as a standby replacement for the computer 15. When the analog controller is not being used, its output may be maintained equal to the feedback signal developed by the potentiometer orrbridge circuit so as to minimize undesirable transients when switching from computer-control to analogcontrol operation.

The above description of the invention is intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art and these can be made without departing from the spirit or scope of the invention as set forth in the claims.

I claim:

. 1. An integrator for producing an output signal which is representative of the time integral of an electrical input signal, said integrator comprising, in combination, a solenoid, terminal means connected to said solenoid for receiving said input signal, a plunger for said solenoid, said plunger being made of magnetic material, means for causing said plunger to apply compression to a fluid in response to said input signal, said compression means including restrictor means for restricting the flow of said fluid to allow said fluid to flow and escape from said compression at a controlled rate so that the position of the plunger is determinative of said time integral.

2. Apparatus as in Claim 1 in which said restrictor means is adapted to render said fluid flow substantially turbulent.

3. Apparatus as in claim 1 in which said integrator includes a housing with said plunger and a liquid in it, and a passageway in said housing communicating between opposite ends of said plunger.

4. Apparatus as in claim 3 in which said passageway causes said liquid to bypass said plunger in its flow between said opposite ends of said plunger, and including valve means for adjusting the rate of liquid flow attainable in said passageway.

5. Apparatus as in claim 3 in which said passageway includes a space between said plunger and an internal wall of said housing, and means for maintaining a substantially constant spacing between said wall and said plunger throughout the entire length of travel of said plunger in said housing.

6. Apparatus as in claim 1 including indicator means responsive to the. position of said plunger for producing an electrical indicator signal representative of said position.

7. Apparatus as in claim 6 in which said indicator means-is a potentiometer having a wiper member coupled to move with said plunger, the frictional engagement of said wiper member in said potentiometer providing a holding force to hold said plunger in place while no electrical input signal is delivered to said integrator.

8. Apparatus as in claim 6 in which said indicator means includes a pair of coils, and circuit means for energizing said coils with alternating current and providing an electrical indication signal dependent upon the relative inductance of said coils and indicating the position of said plunger relative. to said coils.

9. An integrator for producing an output signal which is representative of the time integral of an electrical input signal, said integrator comprising, in combination, a dashpot means including a piston, said piston being made of magnetic material and being adapted to move in said dashpot means, under the influence of an applied force representative of the input signal, to a position representative of the time integral of the magnitude of said applied force, said output signal being characterized by said piston position, and a solenoid, said dashpot piston forming the plunger of said solenoid.

10. Apparatus as in claim 9 in which said solenoid includes a pair of oppositely wound coils encircling said piston.

11. Apparatus as in claim 9 in which said piston includes at least one permanent magnet.

12. Apparatus as in claim 11 in which said piston is composed of two permanent magnets with a member of high magnetic permeability and low magnetic remanence secured between said magnets.

13. An integrator for producing an output signal which is representative of the time integral of an electrical input signal, said integrator comprising, in combination, means for producing a dashpot eflect, said dashpot means comprising a housing, a viscous liquid in said housing, a piston in said liquid in said housing, said piston being slidably mounted with its sides in fluidtight contact with the intemal walls of said housing, said output signal being characterized by the position of said piston, said piston being made of a permanently magnetized material, said piston having a hole communicating between l the ends of said piston, an orifice partially closing one end of housing adjacent said other piston end, two coils wound around said housing adjacent said piston, said coils each being wound in senses opposite to one another, and means for conducting said electrical input signal to said coils. 

1. An integrator for producing an output signal which is representative of the time integral of an electrical iNput signal, said integrator comprising, in combination, a solenoid, terminal means connected to said solenoid for receiving said input signal, a plunger for said solenoid, said plunger being made of magnetic material, means for causing said plunger to apply compression to a fluid in response to said input signal, said compression means including restrictor means for restricting the flow of said fluid to allow said fluid to flow and escape from said compression at a controlled rate so that the position of the plunger is determinative of said time integral.
 2. Apparatus as in Claim 1 in which said restrictor means is adapted to render said fluid flow substantially turbulent.
 3. Apparatus as in claim 1 in which said integrator includes a housing with said plunger and a liquid in it, and a passageway in said housing communicating between opposite ends of said plunger.
 4. Apparatus as in claim 3 in which said passageway causes said liquid to bypass said plunger in its flow between said opposite ends of said plunger, and including valve means for adjusting the rate of liquid flow attainable in said passageway.
 5. Apparatus as in claim 3 in which said passageway includes a space between said plunger and an internal wall of said housing, and means for maintaining a substantially constant spacing between said wall and said plunger throughout the entire length of travel of said plunger in said housing.
 6. Apparatus as in claim 1 including indicator means responsive to the position of said plunger for producing an electrical indicator signal representative of said position.
 7. Apparatus as in claim 6 in which said indicator means is a potentiometer having a wiper member coupled to move with said plunger, the frictional engagement of said wiper member in said potentiometer providing a holding force to hold said plunger in place while no electrical input signal is delivered to said integrator.
 8. Apparatus as in claim 6 in which said indicator means includes a pair of coils, and circuit means for energizing said coils with alternating current and providing an electrical indication signal dependent upon the relative inductance of said coils and indicating the position of said plunger relative to said coils.
 9. An integrator for producing an output signal which is representative of the time integral of an electrical input signal, said integrator comprising, in combination, a dashpot means including a piston, said piston being made of magnetic material and being adapted to move in said dashpot means, under the influence of an applied force representative of the input signal, to a position representative of the time integral of the magnitude of said applied force, said output signal being characterized by said piston position, and a solenoid, said dashpot piston forming the plunger of said solenoid.
 10. Apparatus as in claim 9 in which said solenoid includes a pair of oppositely wound coils encircling said piston.
 11. Apparatus as in claim 9 in which said piston includes at least one permanent magnet.
 12. Apparatus as in claim 11 in which said piston is composed of two permanent magnets with a member of high magnetic permeability and low magnetic remanence secured between said magnets.
 13. An integrator for producing an output signal which is representative of the time integral of an electrical input signal, said integrator comprising, in combination, means for producing a dashpot effect, said dashpot means comprising a housing, a viscous liquid in said housing, a piston in said liquid in said housing, said piston being slidably mounted with its sides in fluidtight contact with the internal walls of said housing, said output signal being characterized by the position of said piston, said piston being made of a permanently magnetized material, said piston having a hole communicating between the ends of said piston, an orifice partially closing one end of said hole, a first capillary tube mounted in said end of said piston and making a bypass connection between Said hole and the region in said housing adjacent said one piston end, a second capillary tube mounted in the other end of said piston and communicating between said hole and the region in said housing adjacent said other piston end, two coils wound around said housing adjacent said piston, said coils each being wound in senses opposite to one another, and means for conducting said electrical input signal to said coils. 